14.2.4.2 The terrestrial system

The metabolic processes that are responsible for plant growth and maintenance
and the microbial turnover, which is associated with dead organic matter decomposition,
control the cycle of carbon, nutrients, and water through plants and soil on
both rapid and intermediate time-scales. Moreover, these cycles affect the energy
balance and provide key controls over biogenic trace gas production. Looking
at the carbon fixation-organic material decomposition as a linked process, one
sees that some of the carbon fixed by photosynthesis and incorporated into plant
tissue is perhaps delayed from returning to the atmosphere until it is oxidised
by decomposition or fire. This slower carbon loop through the terrestrial component
of the carbon cycle affects the rate of growth of atmospheric CO2 concentration
and, in its shorter term expression, imposes a seasonal cycle on that trend
(Chapter 3, Figure 3.2a). The structure of terrestrial
ecosystems, which respond on even longer time-scales, is determined by the integrated
response to changes in climate and to the intermediate time-scale carbon-nutrient
machinery. The loop is closed back to the climate system, since it is the structure
of ecosystems, including species composition, that largely sets the terrestrial
boundary condition of the climate in terms of surface roughness, albedo, and
latent heat exchange (see Chapter 3, Section 3.2.2).

Modelling interactions between terrestrial and atmospheric systems requires
coupling successional models to biogeochemical models and physiological models
that describe the exchange of water and energy between vegetation and the atmosphere
at fine time-scales. At each step toward longer time-scales, the climate system
integrates the more fine-scaled processes and applies feedbacks onto the terrestrial
biome. At the finest time-scales, the influence of temperature, radiation, humidity
and winds has a dramatic effect on the ability of plants to transpire. On longer
time-scales, integrated weather patterns regulate biological processes such
as the timing of leaf emergence or excision, uptake of nitrogen by autotrophs,
and rates of organic soil decay and turnover of inorganic nitrogen. The effect
of climate at the annual or interannual scale defines the net gain or loss of
carbon by the biota, its water status for the subsequent growing season, and
even its ability to survive.

As the temporal scale is extended, the development of dynamic vegetation models,
which respond to climate and human land use as well as other changes, is a central
issue. These models must not only treat successional dynamics, but also ecosystem
redistribution. The recovery of natural vegetation in abandoned areas depends
upon the intensity and length of the agricultural activity and the amount of
soil organic matter on the site at the time of abandonment. To simulate the
biogeochemistry of secondary vegetation, models must capture patterns of plant
growth during secondary succession. These patterns depend substantially on the
nutrient pools inherited from the previous stage. The changes in hydrology need
also to be considered, since plants that experience water stress will alter
the allocation of carbon to allocate more carbon to roots. Processes such as
reproduction, establishment, and light competition have been added to such models,
interactively with the carbon, nitrogen, and water cycles. Disturbance regimes
such as fire are also incorporated into the models, and these disturbances are
essential in order to treat successfully competitive dynamics and hence future
patterns of ecosystem. It should be noted also that these forcing terms themselves
might be altered by the changes that result from changes in the terrestrial
system.

This coupling across time-scales represents a significant challenge. Immediate
challenges that confront models of the terrestrial-atmosphere system include
exchanges of carbon and water between the atmosphere and land, and the terrestrial
sources and sinks of trace gases.

Prognostic models of terrestrial carbon cycle and terrestrial ecosystem processes
are central for any consideration of the effects of environmental change and
analysis of mitigation strategies; moreover, these demands will become even
more significant as countries begin to adopt carbon emission targets. At present,
several rather complex models are being developed to account for the ecophysiological
and biophysical processes, which determine the spatial and temporal features
of primary production and respiration (see Chapter 3, Sections
3.6.2 and 3.7.1). Despite recent progress in developing
and evaluating terrestrial biosphere models, several crucial questions remain
open. For example, current models are highly inconsistent in the way they treat
the response of Net Primary Production (NPP) to climate variability and climate
change – even though this response is fundamental to predictions of the
total terrestrial carbon balance in a changing climate. Models also differ significantly
in the degree of CO2 fertilisation they allow, and the extent to which CO2 responses
are constrained by nutrient availability; the extent to which CO2 concentrations
affect the global distribution of C3 and C4 photosynthetic pathways; and the
impacts of climate, CO2 and land management on the tree-grass balance. These
are all areas where modelling capability is limited by lack of knowledge, thus
making it crucially important to expand observational and experimental research.
Important areas are interannual variability in terrestrial fluxes and the interplay
of warming, management, and CO2 enrichment responses at the ecosystem scale.
Moreover, these issues must be far better resolved if there is to be an adequate
verification scheme to confirm national performance in meeting targets for CO2
emissions. (See Chapter 3, Sections 3.6.2 and 3.7.1.)

Finally, while progress will be made on modelling terrestrial processes, more
integrative studies are also needed wherein terrestrial systems are coupled
with models of the physical atmosphere and eventually with the chemical atmosphere
as well.